1. Field of the Invention
The present invention relates to a vertical CVD apparatus and CVD method for processing a plurality of target substrates all together to form a silicon germanium film, and particularly relates to a technique used for semiconductor processes. The term “semiconductor process” used herein includes various kinds of processes which are performed to manufacture a semiconductor device or a structure having wiring layers, electrodes, and the like to be connected to a semiconductor device, on a target substrate, such as a semiconductor wafer or a glass substrate used for an LCD (Liquid Crystal Display) or FPD (Flat Panel Display), by forming semiconductor layers, insulating layers, and conductive layers in predetermined patterns on the target substrate.
2. Description of the Related Art
Conventionally, for example, poly-silicon is used for gate electrodes of transistors. Gate electrodes of poly-silicon can be easily depleted, when supplied with a bias voltage. As the thickness of gate insulating films decreases, this becomes prominent, which is one of the causes of device properties being deteriorated. In order to solve this problem, it has been studied to employ silicon germanium, which has a higher dopant activation rate, in place of silicon. For example, Jpn. Pat. Appln. KOKAI Publication No. 2003-77845 (patent publication 1: see FIG. 1 and Paragraph 17) discloses a method for forming a silicon germanium film on the surface of a semiconductor wafer. This method employs a vertical heat-processing apparatus, which supplies mono-silane (SiH4) gas and mono-germane (GeH4) gas to form a silicon germanium film by CVD.
FIG. 9 is a view showing a conventional vertical CVD apparatus for forming a silicon germanium film, disclosed in the patent publication 1. As shown in FIG. 9, this vertical heat-processing apparatus includes a reaction container 11 formed of an outer tube 14 and an inner tube 15. A wafer boat 17 for holding semiconductor wafers W is disposed in the reaction container 11. A heater 16 is disposed around the reaction container 11. The reaction container 11 is exhausted by an exhaust section 18.
Four injectors 12a to 12d with different lengths for supplying mono-germane are disposed beside the wafer boat 17 in the reaction container 11. An injector 13 is disposed at the bottom of the inner tube 15 for supplying a mixture gas prepared by mixing mono-silane and mono-germane. It should be noted that the patent publication 1 includes an error in FIG. 1, concerning the injectors 12a to 12d. 
The source gases supplied from the injectors 12a to 12d and 13 flow upward within the inner tube 15, and then enter the gap between the inner tube 15 and outer tube 14 from the top and exhausted from the bottom. As compared to mono-silane and mono-germane with each other in decomposition reactivity, mono-germane is higher than mono-silane in decomposition reactivity, so mono-germane is consumed more. Accordingly, if only the injector 13 is used, the wafers end up having larger values in the thickness of a silicon germanium film and germanium concentration, as the position of the wafers is closer to the supply port of the injector 13. For this reason, this apparatus employs the four injectors 12a to 12d with different heights for supplying mono-germane to compensate for a shortfall of germanium on the downstream side.